Also, wouldnt an understanding that materializes as one gets older of the back story into each memory story (with the frame that memory stories here as having a positive impact) have the potential to change the frame of the memory, which decreases the likelihood of it being remembered? As in, certain memories are pretty great until ypu understand how it came to be that way, in which case either the back story becomes more signigicant or it becomes more mediocore than remembered. And mediocrity is easily forgotten...
I'd also suppose that as people move on with their lives, people relate to each other via shared stories... family stories have little meaning to those not involved, and so have little impact in "fitting in" in the rest of society. As that dawns on individuals leaving home, so are such stories repressed until they more actively need to be remembered.
I am skeptical that "short term memory for family pictures" is a discrete cogntive function.
I'm even more skeptical that you can extrapolate anything generally useful about cognitive decline, since this result is exactly what I'd expect if there were no cognitive decline -- people are better at remembering things they can contextualize to the familiar, so of course they'll be able to remember new information about their families better when they still live with them than after they move out.